1. Field of the Invention
The present invention relates to an optical information reproducing apparatus which reproduces information recorded on an optical disk with a laser beam. More particularly, this invention relates to an optical information reproducing apparatus which can reproduce information contained on various kinds of disks having different pit recording densities.
2. Description of the Related Art
Optical disks, such as a compact disk, a laser disk and a CD-ROM, each have many pits for recording audio data and video data. Both types of data are reproduced by accessing the pits with the reproducing optical unit (hereinafter called "pickup") of an optical information reproducing apparatus (hereinafter called "player"). The player irradiates a laser beam, generated by the pickup, along the pits while scanning a disk in the radial direction with the pickup. The pickup has a sensor to detect the pits based on the intensity of the reflected light of the irradiated laser beam.
Pits may be recesses formed in a disk. When a laser beam is irradiated on a disk region between pits, the sensor receive most of that laser beam as reflected light. When a laser beam is irradiated on the bottom of a pit, the light reflected from the bottom of the pit is dampened by the reflected light from a land portion around the pit due to the phase difference between those two reflected lights. The decrease in the intensity of the light reflected from the bottom of the pit, is detected by the sensor specifically as light from a pit area rather than from a non-pit area.
The depth of a pit (the distance between the surface of the land portion and the bottom of the pit) influences the amplitude of a reproduced signal reflected back along the incident path of the irradiated laser beam. The amplitude of the reproduced signal reaches a maximum when the degree of the cancellation of the reflected light from the pit is maximized. This maximum cancellation occurs when light is reflected from pits having a depth of .lambda./4n with a phase difference of .pi.. As used here, ".lambda." is the wavelength of a semiconductor laser beam produced by the pickup and "n" is the refractive index of the material of the optical disk with respect to the laser beam having the wavelength .lambda. (n being about 1.58 for polycarbonate).
A push-pull type tracking servo system is commonly known as one type of tracking servo system that can trace a row of pits with the pickup. The pit detection by this push-pull tracking servo system however does not work when the pit depth is .lambda./4n. This is due to the way the push-pull type tracking servo system dampens the intensity of the reflected light. When light is reflected from a pit having a .lambda./4n depth, the difference between the intensity of the reflected light from a pit and that of the reflected light from a land portion (which is converted into a tracking signal) becomes zero. When the intensity of the reflected light is zero, there is effectively no tracking signal for the servo system to obtain. Consequently, the push-pull type of tracking servo system would be unable to compensate for any deviation to the laser beam.
In view of the above, ordinary disks are so designed that the depth of pits is slightly shallower than .lambda./4n. For example, with disks of this type, the amplitude of the reproduced signal can be maximized using lasers producing beams having a wave length .lambda. of 633 nm. Other commonly available players are designed to reproduce data from disks having laser beams that utilize wavelengths .lambda. of 830 nm. With the laser type of laser beam, the depth of pits may be as shallow as 633/830 (0.76).times..lambda./4n. This ensures the push-pull type beam tracking control.
The beam wavelength .lambda. of a laser equipped in the pickup and the numerical aperture NA of an objective lens are determined in accordance with the packing density of a disk. When disks having a high packing density are used, it is necessary to reduce the size of the laser beam spot in order to improve the resolution of the optical pickup. In general, the beam spot size is proportional to the wavelength .lambda. of the beam and is inversely proportional to the numerical aperture NA of the objective lens. Generally, therefore, a pickup having a short wavelength .lambda. and a large numerical aperture NA is used to reproduce data from a disk with a high packing density. Likewise, a pickup having a long wavelength .lambda. and a small numerical aperture NA is used to reproduce data from a disk with a low packing density.
This is common for example with CD-ROMs having digital data recorded by EFM (Eight to Fourteen Modulation). The pickup used in this type of CD-ROM utilize standardized laser generators that produce beams with a wave length of 780 nm. In addition, the objective lenses in these devices have a numerical aperture of 0.45.
Most recently, the trend in laser optics has been to decrease the oscillation wavelength of lasers used to reproduce data from optical disks. Laser generators have been developed which produce wavelengths shorter than 780 nm. Corresponding attempts to improve the packing density of optical disks have sought to take advantage of this tendency toward the increasingly smaller wavelengths.
It would be desirable, for example, that players designed to handle disks having a high packing density be able to reproduce data from disks having normal packing density (e.g., those having a recording linear velocity of 1.4 m/s and a track pitch of 1.6 .mu.m). Currently, disks utilizing normal packing density are produced to be compatible with pickups of players specifically designed for those types of disks. Should a player, designed to reproduce data from a disk having a high packing density, be used to reproduce data from a disk having a normal packing density, the reproduced signal would most certainly exhibit poor signal characteristics.